It's fundamentally the same device - a coin cell with a flashing LED, and a switch connects power when water is low. But I have improved the sensor part and used a nice PCB.

The sensor is a sealed unit that is mostly an expanded foam floater, with a tilt sensor inside. The sensor acts as a switch. The whole floater assembly is waterproof-sealed. From Sparkfun's description of the tilt sensor:

"Inside the can are a pair of balls that make contact with the pins when the case is upright. Tilt the case over and the balls don’t touch, thus not making a connection."

The floater is anchored on one side so that when floating in a full tank the tilt sensor will not trip. But when the water level is low, the sensor "hangs" from the wire and the tilt sensor trips.

Here it is installed on the machine. It still requires a small hole in the water tank lid, but I have a plan to make a new lid with the sensor all as one piece.

Here I laser cut the top out of black acrylic:

And the finished product! When the battery need changing (which hasn't happened yet!) or for some reason we need to get to the PCB, just lift off the top.

I wonder if these might make a good kit. I can't be the only one who finds managing the water level of the machine to be troublesome!